Development of an ultraviolet lidar for measurement of tropospheric radio refractivity

Deterministic modelling of tropospheric radio propagation and exploitation of variations of microwave radar propagation requires high quality, high resolution refractivity profiles. Raman lidar offers a ground-based remote-sensing technique for measuring water vapour and temperature. It is capable of continuous monitoring of refractivity parameters, and thus does not suffer from the time sampling bias of statistics derived from radiosonde measurements. In addition, lidar (particularly ultraviolet) remote sensing avoids the problem of EM emissions from radiosonde systems and so can be used under EMCON conditions. In the longer term, such a ground-based or ship-based, remote sensing system offers lower running costs and is less manpower intensive than a radiosonde system, and could prove to be a useful monitor of propagation conditions in the field. A lidar developed at RAL has been installed at the Chilbolton Observatory for the measurement of water vapour and temperature profiles in the troposphere. The wavelength to be transmitted 355 nm, generated by a frequency-tripled Nd:YAG laser, has been chosen after detailed consideration of factors affecting both safety and performance.